NEW YORK — It’s an old saw that murders spike during hot summers, when city dwellers flee their apartments for the streets and tempers soar along with the mercury.

Less, however, has been said about the effect of extreme cold spells on mankind’s capacity for violent crimes.

As of 6:20 a.m. Friday, New York City, with temperatures dropping as low as 11 degrees in recent days, had been murder-free for about 221 hours, a period of more than nine days. The cold, perhaps, pacified a city accustomed, on average, to more than a murder a day.

‘’We’re rooting for more cold weather,” Police Commissioner Raymond W. Kelly said when asked about the streak of murder-free days.

Murders are inevitable in a city of more than 8 million people, Kelly said, so “any respite in that is obviously a welcomed thing.”

The last time more than a week went by without a homicide in the city was three months ago. Hurricane Sandy’s destructive force appears to have quelled man-made violence for an eight-day period, during which the police did not report a single homicide, the Police Department’s chief spokesman, Paul J. Browne, said.

In August 2010, despite the summer heat, seven days passed without a murder. In 2009, there was a six-day reprieve in February and March. In other years, New York considered itself lucky to go five days without a homicide.

A correlation between cold weather and a drop in violence undoubtedly exists, according to several academics whose habitats range from sun-drenched Miami to frostbitten Iowa.

‘’Some have argued that there is something about cold that actually inhibits aggression — literally the effect that cold has on the brain,” said Ellen G. Cohn, a professor of criminal justice at Florida International University.

She added, however, that she believed cold reduced violence primarily for a different reason: Fewer people are likely to be on the streets, which, she said, means “victims and offenders are less likely to come into contact with each other.”

Craig A. Anderson, a psychology professor at Iowa State University, said he, too, believed that some of the decrease had “to do with people probably hunkering down inside” during cold spells.

But he noted that the cold, even as it suppressed street violence, could lead to an increase in domestic violence, which largely occurs indoors. And he observed that some research actually suggested that uncomfortable levels of cold could increase people’s irritability and aggression, just as heat does.

Matthew Ranson, who studied the effect of temperature on crime as a graduate student at Harvard University, said that cold affected crime unequally. Property crime, said Ranson, now a policy analyst, dropped precipitously whenever temperatures fell below a certain point in the 40s. But violent crime, he said, declined more gradually, in a linear manner.

The temperatures were uneven over the recent murder-free period, which began shortly after 1 a.m. on Jan. 16, after a gunshot victim died at a hospital in Brooklyn. The low temperatures for Jan. 16 and the following four days were mostly in the low 30s, and dropped to as low as 11 degrees.

But the streak of murderless days in New York City may have ended Friday morning, when the police found an unconscious woman in her 40s lying outside a building in Brooklyn at 6:20 a.m. She was naked from the waist down, the police said, and died. Her death remained under investigation and had not been classified as a homicide by Friday night.